From www.mutopiaproject.org

The Piano Trio in B-flat major, Op. 11, was composed by Ludwig van Beethoven in 1797 and published in Vienna the next year. It is one of a series of early chamber works, many involving woodwind instruments because of their popularity and novelty at the time. The trio is scored for piano, clarinet, and cello. The key of B-flat major was probably chosen to facilitate fast passages in the B-flat clarinet, which had not yet benefited from the development of modern key systems. Beethoven dedicated the piece to Countess Maria Wilhelmine von Thun.
The above text from the Wikipedia article "Piano Trio No. 4 (Beethoven)" text is available under CC BY-SA 3.0.